Lightweight template rendering for rake tasks, background jobs, etc. using Tilt template engine.

atom.xml.builder

# Feed URLs need to be explict passed as options because there is no request object
atom_feed(
  id: "tag:weblog.rubyonrails.org,2005:/feed/atom",
  root_url: "http://weblog.rubyonrails.org/",
  url: "http://weblog.rubyonrails.org/feed/atom.xml"
) do |feed|
  feed.title("Riding Rails")
  feed.updated(@posts.first.updated_at)

  @posts.each do |post|
    feed.entry(post,
      id:  "tag:weblog.rubyonrails.org,2005:#{post_path(post)}",
      url: "http://weblog.rubyonrails.org#{post_path(post)}"
    ) do |entry|
      entry.title(post.title)
      entry.content(post.body, type: 'html')

      entry.author do |author|
        author.name("DHH")
      end
    end
  end
end

rendering.rb

require 'builder'
require 'tilt'

# NOTE: if your view contains calls to `render` or anything request related
# it will probably blow up. This can be worked around by overriding both
# `render` and `request` in the anonymous context class. Also rendering
# forms for caching will probably fail due to invalid CSRF tokens. This can
# also be worked around by disabling the CSRF token in the form and then
# adding it back by submitting via an $.ajax request.

context = Class.new do
  include ActionView::Helpers
  include Rails.application.routes.url_helpers
  # Additional application helper modules can go here

  # Sets up instance variables for the template
  def initialize(assigns = {})
    assigns.each{ |key, value| instance_variable_set(:"@#{key}", value) }
  end

  protected

  # Needed if you want to use *_url helper methods
  def default_url_options
    { host: 'weblog.rubyonrails.org' }
  end
end

template = Tilt.new("atom.xml.builder")
output = template.render(context.new(posts: Post.feed))